Over long, straight stretches of busy highways, construct heavy canvas coverings so that there is a tunnel for each direction. Periodically throughout the tunnel, place large fans that blow the air in the direction of the traffic. The strong tailwind in the tunnel reduces drag on the vehicles, which
is prominent at highway speeds, resulting in higher fuel economy.

I'd say just skip the fans. You don't want the air going faster than the cars. There will be losses to friction with the canvans and road as well as with the moving cars. The most efficient speed for the air would be some fraction of the traffic speed. When there are no cars present, the ideal speed it 0. When there is one car avery 20 seconds, the ideal speed is probably just a little over 0. If the traffic is bumper to bumper moving at 60 mph, you probably want the air moving pretty close to 60 mph depending on how smooth the walls are. Luckily, the more traffic there is, the faster the air will move without any fans needed.

Someone could study this in an exsisting tunnel to see what wind speed develops and what efficiency gain is experienced by vehicles, but I'd have a hard time imagining that it would be worth the cost of constructing the tunnel. Using canvas might be cheaper to start with, but with the upkeep and replacement I doubt it would be very cost effective. A transparent roof/walls would be nice to avoid the need for lighting during the day, but would need more frequent cleaning. A concrete tunnel would be nice to contain the noise and reduce maintenance, but costs more and makes expanding the road even harder.

That's a good point; in addition to starving drivers of
oxygen, it will also starve engines of same, decreasing fuel
efficiency.

I was going to point out that this idea assumes the
existence of 'canvas' possessing a heretofore unknown
degree of long-term durability, but the longer I linger, the
more things that are just fundamentally wrong with this
idea become evident. Please try again.

I wondered a few times while driving through long
tunnels in Europe (St Gotthard 16km; Frejus 12km;
Laerdal 24km) whether the oxygen levels were
retarding fuel economy. It makes sense to reduce
oxygen levels a little, to reduce the risk of fire in a
tunnel.

Cars that are overpowered and have a throttle (which is still most of them) could possibly benefit from reduced pumping losses, as the throttle would be opener. Possibly. And each car + driver wiped out represents a *huge* fuel saving.

Why has this not been [MFD]? I thought artificial tailwind assistance had been done a few times already here.